This invention relates to electric motors, and more particularly to a protective panel circuit adapted to be used in conjunction with existing manually operated reversing and non-reversing motor controllers.
Manually operated motor controllers have been used successfully for many years to start, reverse, and control the speed of both direct current and alternating current motors. In the case of direct current motors, they limit the initial inrush current during start-up, and can provide incremental steps of either armature or field resistance for speed control. If required, reversal can be accomplished by reversing the armature current. In the case of alternating current motors, motor controllers also limit the inrush current, and when used with wound rotor motors can provide incremental steps of resistance in the rotor circuit for limited speed control.
Depending upon the type of control desired, such motor controllers, or motor control circuits, may be connected to the armature winding, the field winding, or both, in the case of d.c. motors, or the rotor, the stator, or both, in the case of a.c. motors. As used herein, whichever winding is connected to the motor controller is encompassed by the term "motor power circuit".
Motor controllers are generally voltage reducing devices and are provided with an off, or open circuit, position. Such controllers may comprise, for example, step-down autotransformers, variable impedance means, and variable resistance means. However, the term motor controller as used herein is broad enough to include a knife-blade switch in series with the motor power circuit.
If, after the motor is up to speed, a power failure should occur, or if a circuit breaker controlling power supplied to the motor circuit should open due to a temporary overload on the motor, the motor will stop with the motor controller in its position of minimum voltage reduction. Should power then be returned to normal, or the circuit breaker be re-closed due to the removal of the overload condition, the motor may be damaged by excessive initial current.
In addition, as is evident, there exists quite a hazard to personnel and equipment if a motor is permitted to start up suddenly. For this reason, many prior art motor controllers were provided with means by which the controller could be manually brought to the off or open position, thereby preventing the motor from starting suddenly due to the unexpected return of power to the motor circuit. However, such means were sometimes overlooked or forgotten.
In the case of small motors, and where reversal or speed control is not required, a spring return feature on the motor controller was sometimes used to automatically return the controller to the off or open circuit position when power to the motor was interrupted. This was by no means fool-proof because wear, friction, and burned contacts often prevented a full return to the off position.
It is the object of this invention to provide a simple and inexpensive protective panel circuit which can be connected to a motor whereby said motor cannot be re-started after de-energization unless the motor control circuit for said motor is in its off, i.e. open, position.